Laser techniques have been developed that make it convenient to obtain various fundamental frequencies of coherent laser light by utilizing solid, gas, and liquid media. Outstanding among these are solid-state lasers, because they are small, inexpensive, and require no maintenance; however their output is limited to the near-infrared region of the spectrum and is of low power. However, in many applications, laser light having frequencies above those conveniently obtainable is required. Nonlinear optical crystals have, therefore, frequently been employed to convert coherent laser light of a fundamental frequency into laser light of the second harmonic, that is to say, laser light with a frequency twice the fundamental frequency. This conversion is termed "second-harmonic-generation" (SHG).
In the prior art, monocrystalline forms of potassium dihydrogen phosphate (KDP), ammonium dihydrogen phosphate (ADP), barium sodium niobate (BaNaNbO.sub.3), and lithium niobate (LiNbO.sub.3) have been used for generating higher frequency harmonics. Monocrystalline KDP and ADP, while offering greater resistance to optical-irradiation-induced surface damage due to laser beam bombardment, do not exhibit large optical nonlinearities. This rendered these crystals unsuitable for higher harmonic frequency generation by conversion of light derived from low-power sources. In contrast, BaNaNbO.sub.3 and LiNbO.sub.3 show larger nonlinearities but, unfortunately, a low resistance to optical damage. In this regard, the term "resistance to optical damage" means the number of times (shots) the surface of a crystalline material can suffer bursts of laser radiation of a given power density in watts per unit area before the subject crystal shows signs of opacity. Thus, a crystal showing high resistance can sustain a larger number of shots than a crystal of low resistance for the same power density of the incident laser beams. These niobate materials are also mechanically and thermally fragile, and are marginal in SHG efficiency at power levels that are characteristic of laser diodes.
The possibility of using organic molecules in nonlinear optical devices has generated much interest recently because a large number of molecules are available for investigation. Some substituted aromatic molecules are known to exhibit large optical nonlinearities. The possibility of such an aromatic molecule having large optical nonlinearities is enhanced if the molecule has donor and acceptor groups bonded at opposite ends of the conjugated system of the molecule. The potential utility for very high frequency application of organic materials having large second-order and third-order nonlinearities is greater than that for conventional inorganic electro-optic materials because of the bandwidth limitations of inorganic materials. Furthermore, the properties of organic materials can be varied to optimize mechanical and thermo-oxidative stability and laser damage threshold.
U.S. Pat. No. 4,199,698 discloses that the nonlinear optical properties of 2-methyl-4-nitroaniline (MNA) make it a highly useful material in nonlinear devices that convert coherent optical radiation including a first frequency into coherent optical radiation including a second, typically higher, frequency. The nonlinear devices have means for introducing coherent radiation of a first frequency into the MNA and means for utilizing coherent radiation emitted from the MNA at a second frequency.
U.S. Pat. No. 4,431,263 discloses that diacetylenes and polymers formed from diacetylenic species, which are amenable to close geometric, steric, structural, and electronic control, provide nonlinear optic, waveguide, piezoelectric, and pyroelectric materials and devices. Diacetylenes which are crystallizable into crystals having a noncentrosymmetric unit cell may form single crystals or, if they do not, may possibly be elaborated into a polar thin film upon a substrate by the Langmuir-Blodgett technique. Such films often may be polymerized either thermally or by irradiation for use in nonlinear optical systems. Diacetylenes are covalently bonded to substrates through the employment of silane species and subsequently polymerized to yield nonlinear optic devices asserted to have high structural integrity in addition to high efficiencies and optical effects.
Other U.S. patents relating to non-linear optical properties of organic materials include U.S. Pat. Nos. 4,208,501; 4,376,899; and 4,579,915.